how does a High Limit switch work?

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pjstock

New Member
Aug 5, 2021
3
toronto, canada
I am retrofitting a used Osburn 1800 wood insert .
when I plug in the fan unit to test it, at room temperature of about 75F, it does not turn on.
investigating further there is a small High Limit Switch (the size of a dime ) which does not have continuity.
when I bypass that switch and apply power directly to the fan motor, it works fine.

so, is my high limit switch defective?
a quick read suggests that a HLS should Open and Close at certain temperature points.
would it be normal for a HLS to be OPEN at a low temp, like 75F? does it CLOSE at a high temperature?


hmmm, reading a bit further, it seems there might be three TWO temperture points in the switch.
1. below which the switch stays OPEN so the fan does not function and blow cold air.
2. when the heat gets to a warm enough point it closes so that fan is blowing warm air
3. a very high temperature at which it OPENS again to turn the fan OFF. IGNORE THIS THIS MAKES NO SENSE as has been pointed out. I
must have misread something

does that sound about right?
If so, then it would be normal that my fan would not work when just plugged in at 75F ambient temperature.....
 
Last edited:

bholler

Chimney sweep
Staff member
Jan 14, 2014
26,076
central pa
Yes it is normal to be open at room temperature. Not sure why it would open again at high temp that makes no sense
 
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pjstock

New Member
Aug 5, 2021
3
toronto, canada
Yes it is normal to be open at room temperature. Not sure why it would open again at high temp that makes no sense
perfect .
and I agree. I must have misunderstood that 3rd set point, the High temperature shut off idea makes no sense.
thanks
I'll try it all again when I reassemble and actually fire up/heat up the unit.
 

pjstock

New Member
Aug 5, 2021
3
toronto, canada
I'm back.
we have installed the wood burning insert but with the unit fueled and burning and fairly hot, the fan does not kick On (as I would expect it would at a certain heat)
removing the fan unit and jumping over the thermal limit the fan runs fine without the switch.

which leads me back to wondering if the limit switch is bad.

is there a way to test a limit switch?
 

begreen

Mooderator
Staff member
Nov 18, 2005
90,457
South Puget Sound, WA
The fan will not come on until the lower part of the insert is hot. How long have you been waiting for it to come on and with how large of a fire? (Insert temp?) The fan should come on after a good sized, 5-6 split fire has been burning for an hour or so with the face reaching over 500ºF. If it doesn't, sometimes the issue is with the thermostatic snap switch not making good contact with the bottom of the stove. That's the first thing to check.
 

EbS-P

Minister of Fire
Jan 19, 2019
1,457
SE North Carolina
Does it have speed control knob? Some of those switch off full when turned counter clockwise.
 

brenndatomu

Minister of Fire
Aug 21, 2013
6,552
NE Ohio
I bought a nice used Drolet 1400i stove pretty cheap because "the blower doesn't work"...I suspected that it just needed to be hot to work...and it did...I then added a manual switch to the circuit so I can make it run all the time, if I want...it does take a while for the factory switch to come on...and it may not come on at all if the stove is just fired up for one load, especially if there is a few inches of ashes insulating the front/bottom part of the stove near the switch...
 
Last edited:

begreen

Mooderator
Staff member
Nov 18, 2005
90,457
South Puget Sound, WA
Occasionally we get a new stove reported with this issue and find that the snap switch has shaken loose from its clip during transit.